1. Field of the Invention
The present invention relates to a focus detecting device, particularly to a device which detects the focus of an image formed with an optical system by evaluating the sharpness of the image on the basis of a signal obtained by scanning the image with an image sensor.
2. Description of the Prior Art
Various kinds of focus detecting devices of the so-called image sharpness detecting type have been proposed. For example, U.S. Pat. No. 4,047,187 (filed Mar. 31, 1975, issued Sept. 6, 1977, granted to Mashimo et al. entitled "SYSTEM FOR EXPOSURE MEASUREMENT AND/OR FOCUS DETECTION BY MEANS OF IMAGE SENSOR" (corresponding to DT-OS No. 25 14 230) and assigned to the same assignee as this application, proposes a device in which an image formed by an optical system is scanned by an image sensor composed of independently arranged photoelectric transducing elements known as a CCD photosensor, BBD photosensor or photodiode array and so on, to produce an image scanning signal. The latter is passed to a brightness variation detecting circuit by which circuit a signal corresponding to the brightness variation in each between the two adjacent image elements at each scan.
The sharpness of the object image is represented by thus obtained integrated output. The device in accordance with U.S. Pat. No. 4,047,186 is excellent in that a signal precisely corresponding to the imaging condition can be obtained with the image sensor so as to detect the image sharpness with high accuracy.
It goes without saying that even the device in accordance with U.S. Pat. No. 4,047,187 still has room for improvement, for example, with respect to the detection accuracy. For example, as is known, a photoconductive element such as a CdS and so on, can produce a non-linear brightness to output characteristic by selecting the .gamma. of the photoelectric material so it is not equal to 1, and it is possible to detect the image sharpness by making use of this non-linear brightness to output characteristic. On the other hand, as is generally well known, the .gamma. of the photoelectric transducing elements of the image sensor is equal to 1. In consequence, much of the further improvement of the detecting accuracy can be expected for example in the device in accordance with U.S. Pat. No. 4,047,187 by introducing a process for non-linearly transforming the signal representing a brightness variation in each between the adjacent two image elements, while taking the effect of the non-linear brightness to output characteristics of the photoconductive element such as CdS into consideration.